Heretofore, lithium intercalation compounds such as LiMn.sub.2 O.sub.4 have been used in positive electrodes for 4 V secondary lithium and lithium ion batteries. The spinel LiMn.sub.2 O.sub.4 intercalation compound was first obtained by Wickham and Croft by heating lithium carbonate and manganese oxide in 1:2 lithium to manganese molar ratio. D. G. Wickham and W. J. Croft, J. Phys. Chem. Solids, 7, 351 (1958). Wickham and Croft also reported that using an excess of lithium in the reaction mixture led to formation of Li.sub.2 MnO.sub.3, while an excess of manganese led to a mixture containing Mn.sub.2 O.sub.3. These two compounds are intermediate products of the solid state chemical reactions which take place during the high temperature spinel synthesis of the spinel LiMn.sub.2 O.sub.4 and can be present at any time when the reactions are not fully completed. W. Howard, Ext. Abstr., 7 IMLB, 281 (Boston, 1994)
As demonstrated in U.S. Pat. No. 4,426,253 to Hunter, the acid treatment of LiMn.sub.2 O.sub.4 forms a .lambda.-MnO.sub.2 which can be used in a positive electrode for electrochemical power sources. It was later discovered that the spinel LiMn.sub.2 O.sub.4 could be used as the positive electrode for a secondary lithium cell. Thackery et al., Material Research Bulletin, 18, 461 (1983). Thackery et al. demonstrated that the potential-composition curves have two reversible plateaus, respectively at 4 and 2.8 V versus a lithium electrode.
The effect of synthesis temperature on the electrochemical performance of the secondary lithium cell using the 2.8 V plateau of spinel LiMn.sub.2 O.sub.4 has been described in, e.g., U.S. Pat. No. 4,828,834 to Nagaura et al. Nagaura et al. determined that an optimal synthesis temperature for LiMn.sub.2 O.sub.4 using lithium carbonate and manganese dioxide is in the range of between 430-520.degree. C. Using the 2.8 V charge-discharge plateau, Nagaura et al. also determined that LiMn.sub.2 O.sub.4 compounds having a full width at half maximum between 1.1.degree. and 2.10.degree. of a peak 2.theta. equal to 46.1.degree. by x-ray diffraction analysis using FeK.alpha. rays possess favorable cycling performance when used as the active material in cathodes for secondary lithium cells. Furthermore, Nagaura et al. teaches that spinels having a full width at half maximum less than 1.1.degree. do not possess the desired discharging capacity.
Recently, the effect of the higher synthesis temperature on the reversible capacity of the 4 V plateau was described. V. Manev et al., J. Power Sources, 43-44, 551 (1993) and U.S. Pat. No. 5,211,933 to Barboux et al. Manev et al. determined that the synthesis of spinel LiMn.sub.2 O.sub.4 for secondary lithium cells should be performed at temperatures lower than 750.degree. C. Barboux et al. stated that low temperature processes between 200.degree. and 600.degree. C. yield finer size particles of LiMn.sub.2 O.sub.4, do not affect the capacity of the electrolytic cells, and enhance the cycling behavior of the spinel. The decrease in the capacity associated with the increase in the synthesis temperature at temperatures higher than 800.degree. C. was explained by a significant oxygen loss at temperatures higher than 800.degree. C. Manev et al., J. Power Sources, 43-44, 551 (1993).
In U.S. Pat. No. 5,425,932 to Tarascon, a different approach for employing synthesis temperatures greater than 800.degree. C. was described which involves an additional slow cooling step with a cooling rate slower than 10.degree. C. in order to form a spinel with increased cell capacity. Even though this method may increase the capacity of the cell, it may be the source of considerable nonhomogeneous oxygen distribution in the final product, because the oxygen content is a function of firing temperature. For example, the oxygen content in the bulk may be lower than stoichiometric, while an oxygen rich spinel may form on the surface of the particles.
As described in R. J. Gummow et al. Solid State Ionics, 69, 59 (1994)), an infinite number of high lithium content stoichiometric spinel phases exist with a general formula Li.sub.1+x Mn.sub.2-x O.sub.4 where (0&lt;X&lt;0.33). Gummow et al. also states that an infinite series of oxygen rich defect spinel phases exist with a general formula LiMn.sub.2 O.sub.4+Y where (0&lt;Y&lt;0.5) The possibility that X and Y may have negative values has been described for the ranges -0.1&lt;X&lt;0 for Li.sub.x Mn.sub.2 O.sub.4 in U.S. Pat. No. 5,425,932 to Tarascon and -0.1&lt;Y&lt;0 for LiMn.sub.2 O.sub.4+Y in V. Manev et al., J. Power Sources, 43-44, 551 (1993)). As suggested by Gummow et al. and U.S. Pat. No. 5,425,932 to Tarascon et al., the variation of the lithium and oxygen content are accompanied by considerable variation of the spinel lattice parameters.
The existence of an infinite number of lithium manganese spinel phases and the existence of intermediate compounds, thermodynamically stable in the temperature range of spinel preparation but inactive in the 4 V discharge range, namely Li.sub.2 MnO.sub.3 and Mn.sub.2 O.sub.3, demonstrate that the preparation of highly homogenous spinel compounds is extremely complicated. However, a highly homogenous compound is desirable for positive electrodes of secondary lithium cells to provide high specific capacity and a negligible capacity fade as a function of the number of charge-discharge cycles.